1. Field of the Invention
The present invention relates to method of and apparatus for forming a full natural color picture in accordance with an input picture image data.
Although the present invention can be applied to various printers such as an ink-jet printer, thermal transfer copying printer, electrophotographic printer, etc., the invention will be described hereinbelow with respect to an example of a color ink-jet printer.
2. Description of the Prior Art
The apparatus of this kind reproduces a color picture by controlling a quantity of each ink to be formed in a microarea (picture element--pixel) in accordance with an input picture image data using at least three kinds of inks of cyan (c), magenta (m) and yellow (y), or using four kinds of inks additionally including black (b).
However, for example with respect to a magenta ink, it absorbs certain degrees of the blue and red lights, although it should inherently absorb only the green light component. This is similar to the cases regarding the other inks. In addition, since the inks together blot, unless the color correction or compensation of density data is not performed, the picture having the hue and density which are different from the hue and density to be reproduced would have been formed.
To prevent this drawback, conventionally, the following color correction called masking has been performed in general.
c'=fc (y, m, c) PA1 m'=fm (y, m, c) PA1 y'=fy (y, m, c) PA1 fc (y, m, c)=a.sub.11 y+a.sub.12 m+a.sub.13 c PA1 fm (y, m, c)=a.sub.21 y+a.sub.22 m+a.sub.23 c PA1 fy (y, m, c)=a.sub.31 y+a.sub.32 m+a.sub.33 c
where, y, m and c are input picture data; y', m' and c' are color-corrected density data; and fc, fm and fy are color-correction functions.
Various functions have been proposed with respect to fc, fm and fy, but the following linear masking function is ordinarily used.
On the other hand, in the case of reproducing a full color picture, it has been concluded that it is preferable to use a plurality of inks each having a different density even with regard to the same color in order to enhance the dynamic range of density and to reproduce a delicate quality color feeling at the highlight portion or halftone portion. Namely, it is possible to reproduce a picture having colors of low to high densities by using a low-density ink at the highlight portion and using a high-density ink at the shadow portion.
However, in case of forming a picture using a plurality of inks each having a different density, if the color correction is performed using a conventional method, it is difficult to perform the color correction over the whole range from the highlight portion to the shadow portion due to the difference of characteristics between the high-density ink and the low-density ink, causing deterioration in overall picture quality.
In addition, it is difficult to perform the color correction by way of the same color-correction function with regard to the whole area of color space (coordinates) since the color-correction function itself becomes complicated.